1. Field of the Invention
The invention relates to a method for triggering an exposure device for the photomechanical production of structured surfaces as a copy of an electronically memorized model, in particular for exposing printing plates, in which the exposure device includes at least one light source, one picture generating unit comprising movable, electronically individually triggerable micro mirrors arranged in a grid, and one copying optical element.
The invention also relates to an exposure device suitable for performing the method, having a movably embodied exposure head, which includes a light source, one picture generating unit comprising movable, electronically individually triggerable micro mirrors arranged in a grid, and one copying optical element.
2. Prior Art
Even today, up to 90% of offset printing plates are exposed via fill models with the aid of contact copy technology or in isolated cases with projection systems. This means that before an offset printing plate can be exposed to light, a film model is made. This is done with film exposers and film developing machines developed especially for this purpose.
The method step for preparing the film model disadvantageously increases the time and expense involved in putting images on offset printing plates.
From German Patent Disclosure DE 41 21 509 A1, a device suitable for exposing printing plates is known, which has an elongated light source in the form of a linear arc lamp, a collimator lens, an elongated light modulator comprising electronically triggerable movable micromirrors, and a copying optical element, all these elements being disposed in stationary fashion. The elongated linear form of the light source is used here to attain a higher light yield than in point-type light sources. Consequently, however, the stationary exposure arrangement can expose only a very narrow strip of a printing plate. To expose the complete printing plate, the plate is therefore displaced continuously relative to the stationary exposure arrangement. To assure that the copied data remains stationary relative to the exposed material, the transmission of the data within the mirror array must also be displaced in synchronism with the motion of the printing plate.
The known apparatus has the disadvantage that because of the continuous motion of the printing plate, it can expose only in strips and can therefore utilize only a narrow region of the micromirror array. This leads to excessively long total exposure times. Also, because of the limited length of the micromirror array, as a rule it is not possible to expose the entire length of the printing plate simultaneously. Instead, the printing plate must be exposed column by column and moved back and forth for the purpose. This in turn, however, requires that the entire heavy table on which the printing plate is spread out be displaced with accuracy in the micrometer range. Because of the mass to be moved, this cannot be done arbitrarily quickly with the requisite precision. The result is a further lengthening of the total exposure time.
From International Patent Disclosure WO 95/22787, an apparatus for photomechanically making structured surfaces, in particular for exposing printing plates, is known that has a movable exposure head with a light source, a picture generating unit, and a copying optical element. The movable exposure head is compact in design and relatively light in weight. As a result, it can be positioned with micrometer accuracy. The picture generating unit comprises a liquid crystal screen that is disposed between two polarizers. From this reference an exposure method is also known in which the electronically memorized models are broken down into partial images, and the partial images are shown in succession on the liquid crystal screen and copied onto the printing plate in such a way that they combine into a total copy of the model. To that end, the exposure head is moved with extreme precision from one exposure position to the next between each two partial exposures. For exposing the entire printing plate, however, a great number of partial images have to be exposed. This can result in a very long exposure time.
In principle, the exposure time can be shortened by using higher light intensities for the exposure. Given the necessarily narrow design of the exposure head, however, higher light intensities lead to a no longer tolerable heat burden, especially since for the requisite polarization of the light entering the liquid crystal screen, a polarization foil is used. Such polarization filters admit the portion of the incident light that has the xe2x80x9ccorrectxe2x80x9d direction of polarization, while the remainder with the xe2x80x9cwrongxe2x80x9d direction of polarization is absorbed. A considerable amount of heat is thus created in the polarization filter at high light intensities and this heat must be dissipated if destruction of the polarization filter is to be averted. In the liquid crystal screen itself and in the second polarization foil that follows as well, heat from absorbed light occurs.
Particularly in producing offset printing plates for newspaper printing, the sequential exposure in known methods and apparatuses leads to disadvantageous bottlenecks in terms of time.
It is an object of the present invention to provide an improved exposure device for printing plates of the above-described type, which produces exposure results of satisfactory quality with reduced total exposure times.
It is also an object of the present invention to provide an improved method for controlling an exposure device of the above-described type for exposing printing plates according to an electronically stored model, whereby exposure times may be reduced while still producing exposure results of satisfactory quality.
In a method of this generic type, this object is attained in that the model is broken down electronically into two-dimensional partial images; that the partial images are shown in succession, but each partial image is shown with its entire area at the same time, by means of the picture generating unit, and successive partial exposures of the surface to be structured are made, in that a exposure head, provided with the light source, the picture generating unit and the copying optical element, moves between each two partial exposures from one exposure position to the next and stops there for the exposure, so that the individual partial copies are combined again into a total copy of the electronically memorized model.
The invention adopts the known method, in conjunction with a precision-controlled movable exposure head, which is based on the production of flat partial images using a liquid crystal screen, to a stationary exposure arrangement that is based on linear picture generation by means of micromirrors. For that purpose, in a first step of the invention, the stationary exposure device is embodied as a movable exposure head, and in a second step of the invention the quasi-linear micromirror array is replaced by a genuinely two-dimensional array, and in a third step of the invention the electronic process control for the motion of the exposure head and the exposure of the partial images is transferred. In this respect, the teaching provided in the first paragraph of the background section of German Patent Disclosure DE 41 21 509 A1 actually leads one skilled in the art away from using two-dimensional light modulators. If conversely one skilled in the art takes as his point of departure the apparatus and the associated method known from WO 95/22787, then by simply adopting the stationary exposure arrangement with micromirrors known from DE 41 21 509 A1, he certainly does not arrive at the subject of the invention. Nor does simply replacing the liquid crystal screen with the linear micromirror array lead directly to the invention. That would require a genuinely flat-area, two-dimensional micromirror array, which furthermore must be triggered quite differently from the linear array. Furthermore, the micromirror array cannot simply be used to replace the liquid crystal screen, because it reflects the light and does not transmit it. In addition, an absorber for the unusable reflected light must be provided, while the polarizers are no longer needed.
It can happen that not all the partial images can actually contain picture information. According to the invention, only partial images that have picture contents are therefore transmitted to the surface to be exposed. Because of the reduced number of partial images, the total exposure time is reduced approximately in proportion to the total number of partial images without any picture content. The exposure head arrives at each position of a partial image, along a meandering path.
The total exposure time still required for the remainder of the partial images can be reduced still further if each partial image is assigned a first data set for its image content and the second data set for its position, and that only data sets belonging together with image content are used to generate a succession of control data sets for transfer to a numerical controller. As a result, the times required for changing position between two exposure sites between the individual exposure times can also be reduced.
The time for producing the offset printing plates can be reduced by a further distance reduction in that the quantity of the second data sets is sorted in such a way that the lowest possible sum of individual spacings between the positions of the partial images results, and the data sets are transferred in this order to the numerical controller. In this way, the total image is produced over the shortest possible travel distance.
High travel speeds of the exposure head are attained if at least one electric linear drive is used as the drive for generating a motion of the picture generating unit. The exposure times can directly follow these thus-shortened positioning times without any waiting time, since the effects of elasticity, play and friction and natural vibrations are largely avoided by means of the electric linear drive.
Since offset printing plates can have some waviness that is sometimes greater than 2 mm, and these uneven features are also unevenly distributed, an adequate depth resolution of the copy must be attained by means of suitably small selected apertures. The low light yield, however, lengthens the exposure times for the partial image. To shorten these exposure times, it is provided that before each copying of a partial image, a measurement of the spacing of the picture generating unit or of the exposure head from the surface to be exposed is made, and deviations from a previously input value are automatically corrected. In this way, with a surprisingly low depth resolution, even with wavy plates, a high copy quality can be attained at short exposure times. The correction can preferably be made by moving the entire exposure head. In this way, even offset printing plates of various thickness can be exposed. The focal length can also be corrected accordingly, although this can lead to copying errors.
A uniform copy quality over the entire surface area of the model is attained if for copying a light source is used whose light flux is measured, and deviations from a predetermined light flux are automatically corrected.
This correction can be done either if the correction is done by varying the exposure time or if the correction is done by varying an electrical supply to the light source.
Even the most demanding print models in terms of quality can be produced by this method if the picture generating unit is copied at a reduced size. The reduction in size makes resolutions of 2540 dpi, for instance, possible. However, then the model must also be broken up into a corresponding number of partial images that are exposed in succession and then combined into the total model. Because the number of individual exposures is thus increased, the total exposure time rises considerably.
If a positioning of the partial images is done with an accuracy of better than 5 xcexcm, and in particular better than 2 xcexcm, the individual partial images succeed one another without gaps.
The total exposure time of a model can be reduced by providing that by using a plurality of exposure heads, a plurality of partial images are simultaneously copied.
Collisions of different exposure heads are avoided by providing that a relative motion of a plurality of copies with respect to the pattern carrier is effected synchronously. The spacing of the partial images lined up with one another and copied simultaneously advantageously remains constant then. Since the likelihood that both exposure heads simultaneously lack any picture content drops, and an exposure position can at the same time be skipped, it is advantageous if within a predetermined limit region the exposure heads can move independently of one another.
Because a spacing between two simultaneously exposed copies is variable, models with different resolutions can be exposed using the same exposure device. The spacing between two simultaneous copies amounts to an integral multiple of the individual picture dimensions.
Greater uniformity of the transitions is attained if the partial images are copied overlapping one another. In the region of the overlaps from the edge of the pictures to the interior, the density is adapted to rise linearly, so that by the superposition, a distribution corresponding to the original image is obtained.
An exposure device that is especially suitable for performing the method of the invention is distinguished in that a measuring and regulating device for measuring and regulating the spacing of the picture generating unit or of the exposure head from the surface to be exposed is provided. Before each exposure, by means of the measuring device, the spacing from the surface to be exposed is ascertained. By way of example, known laser measuring devices are suitable as the measuring device. The spacing is determined by interferometric length measurement. However, the space measurement can also be done by known acoustical or optical measuring methods. By comparisons with a predetermined set-point value, a controlling variable is determined that then readjusts the spacing via a suitable final control element. By way of example, this can also be done by adjusting the focal length or by motor-driven adjustment of the height of the exposure head or parts thereof. Because of the regulation provided according to the invention, light sources can be used that sweep a wide angle in space, so that because of the high light yield, short exposure times are possible.
Especially high speeds of motion and accelerations are attained by providing that the printing plate together with a required heavy plate to which it is fastened is not moved and instead only the exposure head is moved. In this way, more than ten partial images per second can be positioned and exposed.
The entire model is copied with constant quality over the entire surface if a sensor for measuring the light flux and a regulating circuit for correcting deviations in the light flux from a set-point value are provided. As a result, even slow changes in the light yield cannot have any influence on the outcome of the work.
For example, changes that make themselves felt over a relatively long period of time can be corrected by providing that an adjusting device for varying an exposure time is provided in the regulating circuit. The sensor for measuring the light flux is preferably disposed in the vicinity of the surface to be exposed, so that the incident light flux is detected once and for all before the exposure of the partial images begins.
Transient changes, caused for instance by fluctuations in the voltage from the power grid, can be compensated for if an adjusting device for varying the light flux is provided in the regulating circuit. Here the light sensor is disposed preferably in the vicinity of the light source, or the supply voltage is measured directly. By means of known circuits, the supply voltage can thus be kept constant, for instance.
It is possible to avoid turning the lamp on and off frequently if a shutter or switching the light flux is provided. This lengthens the service life of the lamp.
If the picture generating unit itself is embodied such that it acts as an exposure shutter, then it is advantageously possible to dispense with a separately embodied exposure shutter.
An especially advantageous dynamic behavior of the overall device is obtained if the driver for generating the motion of the exposure head is embodied as a linear drive. Such drives have high positioning precision of better than 2 xcexcm.
The entire model can be finished in an even shorter time is a plurality of exposure heads are disposed on one axis.
An especially favorable construction is obtained if each exposure head is assigned its own linear drive, and for the other axis a common linear drive for all the exposure heads is provided. In return, the locking speed in one axis is somewhat less than that of the two heads in the other direction. This affects the total exposure time only slightly. Because each exposure head has its own linear drive in the other direction, it is possible to switch briefly over to different resolutions, by moving the exposure heads in such a way as to create a larger or smaller interstice.